This invention relates to several improvements for determining when to initiate a defrost mode for a heat pump, and also to protect associated systems such as a hot water supply system during a defrost mode.
Heating, ventilation and air conditioning (HVAC) systems are utilized to provide cooling and heating in buildings. Typically, a compressor delivers a refrigerant to a heat exchanger which is a heat exchanger associated with the interior of a building. The refrigerant passes to an expansion device downstream of the heat exchanger, and downstream of the expansion device to an evaporator. The evaporator is typically a heat exchanger that exchanges heat with an outside environment.
When an HVAC system is utilized to provide heating, it can be said to be in a heat pump mode. Under such conditions, the evaporator may be in a very cold environment, such as during winter. Problems can arise in that frost can form on the evaporator heat exchanger coils. This lowers the ability to transfer heat from the system to the outside environment through the evaporator heat exchanger.
Thus, such systems have a defrost mode. In defrost mode, the hot refrigerant leaving the compressor is bypassed directly to the evaporator. The bypass can occur by reducing the removal of heat in the heat exchanger, or can be a bypass of some refrigerant around the heat exchanger. To date, there has been little in the way of sophisticated control to determine how and when the defrost mode should be actuated.
Moreover, when a heat pump system is utilized to heat water, such as for a hot water heating system, problems can arise during defrost mode. In particular, defrost mode is often utilized in combination with shutting down the pumping of water through the heat exchanger. This is done since if the water continues to flow, the refrigerant will be cooled in the heat exchanger. Under such conditions, the water that sits in the heat exchanger can boil, which would be undesirable.
Another problem can occur near the end of a defrost mode. At this point, the bulk of the frost will have melted. There are water droplets remaining on the coil. Since the fan is turned off, there is no air removing these droplets. Leaving the droplets on the coil increases the likelihood that the coil will quickly frost again after the termination of the defrost mode. Further, since the fan is not driving air over the coil, little heat is being removed from the refrigerant in the coil. Thus, the refrigerant temperature exiting the evaporator remains higher than might be desired.